西秦岭新生代夷平面发育特征和年代及其意义

青藏高原多圈层相互作用研究一直是国际地学界研究的热点和难点，而高原各主要块体精确的内外耦合作用记录成为取得突破的关键。西秦岭地处高原东北向生长的关键节点部位，夷平面保留完好，具典型性，是研究区域内外力耦合作用的良好载体。在野外考察的基础上，通过地貌因子提取法和目视解译法，对该区进行定量解译分析发现，山顶面与主夷平面分别残留于高山顶部和普遍分布在西秦岭山脉主体部分，夷平面东北向倾斜暗示高原在该区抬升幅度由内部向边缘逐渐减小。梳理和分析该区有关低温热年代学和构造变形证据，获得该区新生代以来造山期次主要发生在66~47 Ma、38~22 Ma、≤13 Ma，期间的47~38 Ma和22~13 Ma相对平静期为准平原过程。初步推断现存的山顶面在始新世中期（约47 Ma）开始夷平，主夷平面的发育始于中新世早期（约22 Ma），约13 Ma准平原形成。青藏运动导致这两级地貌面隆升到现代高度，最终奠定现代地貌格局。

Formation characters and ages of the planation surfaces on West Qinling during Cenozoic era and their significances

Multi-layered interaction of the Tibetan Plateau is elusive and becoming a hotspot of geosciences. The key issue is to make clear the coupling of external/internal forces in each terrane. The West Qing orogenic belt is regarded as the northeasternmost spreading frontier of the plateau growth during the Cenozoic era， and the planation surfaces are well preserved in the mountain tops and argued as the precious coupling records of external/internal forces which related to the plateau formation and evolution. Therefore， to study the planation surfaces of West Qinling can provide the boundary conditions for our understanding the whole growth process of Tibetan Plateau. Mainly based on the geomorphologic factors and visual interpretation techniques， combining with the field investigation and existed geological and morphological information， we first quantitative analyze the levels， distributions and characters of the relict planation surfaces and finally illustrated in a map. The higher level of relict landscapes exists in the mountain summits with altitudes at 4 100 m in the west portion and then gradually changed to 3 600 m in the northeast parts. And the lower level of relict landscapes distributes on the mainly body of West Qinling， with altitudes from 3 600 m in the west portion to 2 700 m in the northeast. The northeastward tilt of these surfaces implying that the uplift amplitude of the Tibetan Plateau is gradually lower from center to margin. Compiling the previous study of low-temperature thermochronology and other morphotectonic evidences， we speculate that the higher and lower landscape remnants developed at low elevation during quiescent periods of orogenic building-up （which mainly occurred at 66~47 Ma， 38~22 Ma and ≤13 Ma）， at the age ranges of middle Eocene and early-middle Miocene， respectively. By synthesis the yielded data here and the existed records of deformation and paleoaltimetry， a regional up-to-date morphotectonic scenario through the Cenozoic is finally reconstructed. Our study demonstrates that far-field stress due to the India-Asia collision propagated to the modern northeasternmost boundary as early as Paleocene. Subsequently， the topography of West Qinling developed through discrete steps of tectonic uplift activity/quiescence or enhanced erosion until the late Miocene， when the modern morphology of northeasternmost plateau finally formed. Typically， the formation of higher and lower planation surfaces occurred at about 47 Ma and 22 Ma， respectively. They should be firstly eroded to low altitude （<1 500 m） during the quiescence time intervals， and finally involved into the present Tibetan morphology by the Qingzang Movement. Our study supports the ideas that the growth of the Tibetan Plateau is synchronously with multiple-stages.